Commercial aircraft gas turbine engines must meet certain federally-mandated smoke and emissions requirements. For example the Federal Aviation Administration (FAA) has a regulation which limits the amount of unburned hydrocarbon emissions including smoke and vapor forms thereof. Furthermore, the International Civil Aeronautics Organization (ICAO) also places limits on emissions including unburned hydrocarbons, oxides of nitrogen and carbon monoxide.
The prior art includes various means for reducing gas turbine engine exhaust emissions including improved carburetors to more fully mix and atomize fuel and air for obtaining more complete combustion. It is known that unburned hydrocarbons will result when combustion, or reaction, process occurs at less than about 1500.degree. F., whereas complete burning of hydrocarbons will occur at reaction temperatures greater than about 2000.degree. F., with reaction temperatures therebetween resulting in varying amounts of hydrocarbons.
However, the combustion process generates such high temperatures in a gas turbine engine combustor that unless the combustor itself is adequately cooled, conventional metallic alloys used in fabricating the combustor will suffer severe thermal distress. Accordingly, conventional gas turbine engines employ means for film cooling the combustor liner to protect the liner from high temperature combustion gases. U.S. Pat. No. 3,978,662--DuBell et al, assigned to the present assignee, discloses several means for providing effective film cooling of a combustor liner. The cooling fluid used for film cooling in a gas turbine engine is compressor discharge air which has a typical temperature of about 1000.degree. F. at rated power, but only 350-400.degree. F. at idle, where most emissions are formed.
The use of a low temperature boundary layer, or film of cooling air along the entire inner surface of a combustor liner provides for effective cooling of the liner from the hot combustion gases. However, since the temperature of that boundary layer is about the temperature of the cooling air, which is substantially less than about 1500.degree. F., quenching, or cooling, of the fuel/air mixture against that boundary layer will occur during operation. Since combustion of the quenched fuel/air mixture along the cooling air boundary layer will therefore occur at temperatures much less than about 1500.degree. F., unburned hydrocarbons and carbon monoxide will be generated.
Prior art combustors typically include carburetors effective for obtaining a fuel-rich center region in the primary combustion zone which has generally short combustor residence time as well as having relatively low recirculation of the fuel/air mixture. This is typically done for preventing entrainment of unburned fuel in the cooling film with resultant quenching of the fuel/air mixture for reducing exhaust emissions.
Depending upon the particular gas turbine engine model, these unburned hydrocarbons, as well as carbon monoxide, may meet the required exhaust emissions requirements. However, in a particular model of a gas turbine engine presently manufactured by the present assignee, more restrictive FAA and ICAO emissions requirements were enacted, thus requiring a change in design to reduce unburned hydrocarbon and carbon monoxide emissions to comply therewith. Inasmuch as the engine is a current production engine, it was desirable that changes be kept to a minimum within the restrictions imposed by a preexisting gas turbine engine design.
Furthermore, a combustor designed for having reduced emissions typically is limited in the amount of emissions reduction obtainable by its ability to obtain acceptable relight of the combustor at altitude when found in a gas turbine engine powering an aircraft. Therefore, reduced exhaust emissions and altitude relight capability of a combustor must be obtained together for an acceptable aircraft engine combustor.